Accessory Respiratory Organs in Fishes | Phylum Chordata

In this article we will discuss about the Accessory Respiratory Organs in Fishes:- 1. Types of Accessory Respiratory Organs 2. Functions of Accessory Respiratory Organs 3. Significance.

Types of Accessory Respiratory Organs:

1. Suprabrachial Organ:

The supra-branchial organ is a specialised type of respiratory struc­ture encountered in Clarias batrachus (Fig. 6.83A).

It has a complex structural organisation and consists of the following portions:

(a) An elaborate tree-like structure growing from the upper end of the second and fourth gill-arches of either side. This dendritic organ is composed of numerous terminal knobs, each has a core of cartilage covered by vascular membrane. Each exhibits eight folds which suggest that one such knob is formed by the coalescence of eight gill-filaments.

(b) There are a pair of highly vascularized supra- branchial chambers within which the tree-like structures are contained. The supra-branchial chambers are developed as the vascularized diverticula of the branchial chamber.

(c) The entrance of the supra-branchial chamber is guarded by ‘fan’-like structures which are developed by the fusion of the adjacent gill- filaments of the dorsal side of the gill-arches.

The supra-branchial organs, like the gills, are lined by thin outer epithelial layers with inter­cellular spaces separated by the pilaster cells. The organs and the supra-branchial chambers are supplied by afferent and efferent blood ves­sels from the gill-arches.

The supra-branchial organs help to breathe in air. The supra-branchial chamber has inhalant and exhalant apertures. These fishes come to the surface of the water and gulp air into the supra-branchial organs. Atmospheric air from the pharyngeal cavity is taken into the supra-branchial chamber by an inhalant aper­ture located between the second and third gill- arches.

After gaseous exchange the air from the said chamber expels into the opercular cavity by the gill-slit lying between the third and fourth gill-arches. The fan-like structures present in the second and the third gill-arches help to intake the air while the expulsion of the air from the supra-branchial, chamber is caused by the contraction of its wall. Thus the supra-branchial chamber and its contained organs function as ‘lung’.

2. Branchial Outgrowths:

In climbing perch (Anabas testudineus) there are two spacious sac-like outgrowths from the dorsal side of the branchial chambers (Fig. 6.83B). The epitheli­um lining these outgrowths is highly vascular and becomes folded to increase the respira­tory area.

Each chamber contains a characte­ristic rosette-like labyrinthine organ. This organ develops from the first epibranchial bone and consists of a number of shell like concentric plates. The margins of the plates are wavy and the plates are covered with vascular gill-like epithelium.

Each branchial outgrowth communicates freely not only with the opercular cavity but also with the bucco­pharyngeal cavity. Air enters into the out­growth by way of the buccopharyngeal open­ing and goes out through the external gill-slits. The entrance is controlled by valves.

Anabas can breathe in air by the help of these organs. These fishes have the habit of migration from one pond to the other. Their overland progression is peculiar and is assis­ted by the operculum and the fins. Each oper­culum bears sharp spines at the free edge.

During travelling the opercula alternately spread out and fix to the ground by the spines and get the forward push from the pectoral fins and the tail. The proverb that the fish can climb the trees seems to be erroneous. The climbing perches are found in the branches of palm or other trees which are possibly brought there by the kites or crows while these fishes migrate over the land.

In Trichogaster fasciatus the accessory respiratory organs are similar to that of Anabas and consist of supra-branchial chamber, labyrinthine organ and respiratory membrane (Fig. 6.84).

The labyrinthine organ is simpler in construction in comparison to that of Anabas. Each organ assumes a spiral configuration with two leaf-like expansions. Each of these two expansions is composed of loose connective tissue which is covered by highly vascular epithelium.

3. Pharyngeal Diverticula:

In the Snake- headed fishes and Cuchia eels, the accessory respiratory organs are relatively simplified. These fishes can survive prolonged drought and their air breathing habit enables them to remain out of water for some time. In both the group of fishes, the pharynx gives a pair of sac­like diverticula for gaseous exchange.

In Channa, the accessory respiratory organs are relatively simpler and consist of a pair of air-chambers (Fig. 6.83D). These are developed from the pharynx and not from the branchial chamber as seen in others.

The air-chambers are lined by thickened epithelium which is highly vascularized. The air-chambers are simple sac-like structures and do not contain any structure. These chambers function as the lung-like reservoirs. In Channa striatus the vas­cular epithelium lining the chambers becomes folded to form some alveoli. The gill-filaments are greatly reduced in size.

In Cuchia (Amphipnous cuchia) the acces­sory respiratory organs consist of a pair of vas­cular sac-like diverticula from the pharynx above the gills (Fig. 6.83E). These diverticula open anteriorly into the first gill-slit. These diverticula function physiologically as the lungs.

The gills are greatly reduced and a few rudimentary gill-filaments are pre­sent on the second of the three remaining gill- arches. The third gill-arch is found to bear fleshy vascular epithelium. In Periophthalmus, a pair of very small pharyngeal diverticula is present which are lined by vascular epithelium.

4. Pneumatic Sacs:

In Heteropneustes fos­sil is, a pair of tubular pneumatic sacs, one on each side of the body, act as the accessory respiratory organs.

These long tubular sacs arise as the outgrowths from the branchial chamber and extend almost up to the tail between the body musculature near the verte­bral column (Fig. 6.83C). In Sacco-branchus, similar tubular lung-like outgrowths of the branchial chamber extend back into the body musculature.

5. Buccopharyngeal Epithelium:

The vascular membrane of buccopharyngeal region in almost all the fishes helps in absorbing oxygen from water. But in mudskippers (Periophthal­mus and Boleophthalmus) the highly vascularized buccopharyngeal epithelium helps in absorbing oxygen directly from the atmos­phere.

These tropical fishes leave water and spend most of the time skipping or ‘walking’ about through dampy areas particularly round the roots of the mangrove trees. The old idea that the mud-skippers use the vascular tail as the respiratory organ is not supported by recent Icthyologists.

6. Integument:

Eels are recorded to make considerable journey through damp vegeta­tion. The common eel, Anguilla Anguilla can respire through the integument both in air and in water. In Amphipnous cuchia and mud- skippers, the moist skin sub-serves respiration.

Many embryos and larvae of fishes respire through the skin before the emergence of the gills. The median fin fold of many larval fishes is supplied with numerous blood vessels and helps in breathing. The highly vascular oper­cular fold of Sturgeon and many Catfishes serves as the accessory respiratory structure.

7. Gut epithelium:

The inner epithelium of the gut essentially helps in digestive process. But in many fishes the gut becomes modified to sub-serve respiratory function. Cobitis (giant loach of Europe) comes above the water-level and swallows a certain volume of air which passes back along the stomach and intestine. In Misgurus fossilis, a bulge just behind the stomach is produced which is lined by fine blood vessels.

The bulge acts as the reservoir of air and functions as the accessory respira­tory organ. After the gaseous exchange, the gas is voided through the anus. In certain other fishes, Callichthyes, Hypostomus and Doras the highly vascular rectum acts as the respira­tory organ by sucking in and giving out water through the anus alternately.

In these fishes the wall of the gut becomes modified. The wall becomes thin due to the reduction of the mus­cular layers.

8. Swim-Bladder acts as Lung:

Swim-bladder is essentially a hydrostatic organ but in some fishes it functions as the ‘lung’. In Amia and Lepisosteus, the wall of the swim-bladder is sacculated and resembles lung. In Polypterus the swim-bladder is more lung-like and gets a pair of pulmonary arteries arising from the last pair of epibranchial arteries.

The swim-blad­der in dipnoans resembles strikingly the tetra- pod lung in structure as well as in function. In Neoceratodus, it is single, but in Protopterus and Lepidosiren it is bilobed. The inner sur­face of the ‘lung’ is increased by spongy alve­olar structures.

In these fishes, the ‘lung’ is mainly respiratory in function during aestiva­tion because the gills become useless during this period. Like that of Polypterus, the ‘lung’ in dipnoans gets the pulmonary arteries from the last epibranchial arteries.

In Notopterus, the swim-bladder becomes more complex and acts as a lung. Except the hydrostatic, sound production and hearing, a new function like respiration was innovated in Notopterus. In Notopterus chitala the posterior tip of swim-bladder is enlarged which is called caudal extension and the ventral part gives off several finger-like projections, the dorsal side of the gas bladder possesses a specialised stria­ted muscle.

The anterior part extends into a projection to the ear. An artery arising from the dorsal aorta forms a network of blood capilla­ries that spread the entire inner surface of the abdominal and caecal parts of the swim blad­der.

The blood capillaries that cover a single epithelial layer helps in the gaseous exchange between the blood and the air of the swim-­bladder. This air breathing habit is considered as a secondary adaptation in these fishes.

Functions of Accessory Respiratory Organs:

The accessory respiratory organs contain a high percentage of oxygen. The fishes possess­ing such respiratory organs are capable of liv­ing in water where oxygen concentration is very low. Under this condition these fishes come to the surface of water to gulp in air for transmission to the accessory respiratory organs.

If these fishes are prevented from com­ing to the surface, they will die due to asphyx­iation for want of oxygen. So the acquisition of accessory respiratory organs in fishes is an adaptive feature.

Further it has been observed that the rate of absorption of oxygen in such organs is much higher than the rate of elimination of carbon-dioxide. Hence, it is natural that the gills excrete most of the carbon-dioxide. Absorption of oxygen appears to be the prima­ry function of the accessory respiratory organs.

Significance of Accessory Respiratory Organs:

The cause of emergence of the acces­sory respiratory structures in fishes in addition to the primary respiratory organ is very diffi­cult to interpret. There are two contrasting views regarding the origin of the aerial acces­sory respiratory structures. First view: some fishes have the natural instinct to make short excursion to the land from the primal aquatic home.

To remain out of water, the develop­ment of certain devices to breathe in air becomes necessary. Second view holds that the fishes are forced to ascend the land when the oxygen content of water falls to a consi­derable extent. The fishes in that particular condition of life gulp in atmospheric air from the land and pass it into the accessory respira­tory structures.

If they are prevented by mechanical barriers to come to surface, the fishes will die of suffocation. This habit of swallowing bubbles of air is observed in many bony fishes, especially living in shallow water which dries up periodically or becomes foul by the decomposition of aquatic vegetation.

As a consequence of the air-breathing habit for a considerable span of time, the fishes have developed specialised accessory respiratory organs in addition to the gills.

Most of such structures encountered in the fishes assume the shape of reservoir of air and originate either from the pharyngeal or branchial cavi­ties. In extreme cases the reservoir may house special structure for gaseous exchange.

However, the development of such accessory respiratory organs is essentially adaptive in nature to meet the respiratory need and thus enables the fishes to tolerate oxygen depletion in water or to live on land over a varying peri­od of time. The development of the accessory respiratory organs depends directly on the ability to remain out of the water.